Amanda's age is being used to describe the other two, Ella and Zoe. Therefore, let the variable, x, represent Amanda's age.﻿In many word problems, whatever comes after the word "than" is usually what you should let the variable represent.﻿

x = Amanda's age
x - 6 = Ella's age {Ella is 6 years younger than Amanda}
2x = Zoe's age {Zoe is twice as old as Amanda}